Quote:
Originally Posted by olddog
Be warned that Ford started with Speed Density. It uses a MAP sensor in the manifold to estimate what the air flow into the engine is at a given throttle position. It does not work well with big cams, as the vacuum can be lower at idle than it is at off idle with such a cam. Second it does not automatically adjust for altitude changes. When you turn the key on the ECU saves the MAP sensor reading and assumes that is the atmospheric pressure. It does not update again until you turn the engine off and restart. So if you drive 4000 feet up a mountain, its no better than a carb, however turning the engine off/on is much easier than changing jets.
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Olddog,
Your statement isn't entirely true. You were good up until you gave the 4000 feet comment.
Speed Density systems actually DO compensate for altitude on the fly without having to turn the key on/engine off (KOEO). They do it by sensing the manifold pressure (via the MAP) every time it sees WOT (via the TPS or Full Throttle switch on a minimum function system).
So on your way up that 4000' mountain, every time you push the throttle to the floor the ECM sees atmospheric pressure (since man vac drops to 0"Hg) and notes it as BARO.
I have seen ONE instance where a customer drove his car over a very long distance with a *gradual* rise in elevation without ever having to floor the gas pedal and he got a CEL out of it. That's one instance in over 25 years of EEC work so it is rare.
Hope That Helps,
Phill Pollard